Methods and device for filtration of exhaust gases for a diesel engine with a filtration surface which is variable by means of controlled obstruction

ABSTRACT

A method in which all or some of the particles contained in the exhaust gases of a diesel engine are retained on particle filters and burnt due to the action of a combustion catalyst. At least a part of the particle filters are obstructed when the temperature θg of the exhaust gases for filtration is equal to or less than a threshold temperature θs, so as to limit or avoid cooling of the obstructed part of the particle filters and to maintain the same at a temperature θo greater than or equal to θs up until the time when θg becomes greater than θs again and thus permit accelerated regeneration of the obstructed part of the particle filters. Also disclosed is an exhaust gas filtration device which permits the carrying out of the filtration method with continuous and regular regeneration of the particle filters.

The present invention relates in general to the field of particulatefilters and, more particularly, to a device for filtration of exhaustgases for diesel engines.

More particularly, the present invention relates to an exhaust gasfiltration device for diesel engines further comprising a variablecapacity filtration means for said gases, in which a catalysis means isdisposed, associated or not with post-injection of diesel and with anexhaust gas recirculation system.

The reduction of pollutant emissions generated by combustion enginesand, in particular, diesel engines is the objective set by theauthorities. For this purpose, the establishment of ever more stringentstandards obliges the automobile manufacturers to develop engines withincreasingly reduced pollutant emissions, in order to limit the releaseof unburnt particles. Since modification of engine combustion is nolonger sufficient to reach these low levels, the additional use ofexhaust gas filtration devices will be indispensable to retain theseparticles and to meet the standards.

Thus, in order to reduce the emission of unburnt pollutant gases and ofsolid particles, automobile manufacturers have developed and havegeneralized the use of catalytic converters or catalysts, generallyconsisting of a stainless steel casing, a thermal insulation, and ahoneycomb support impregnated with precious metals such as platinum orrhodium.

These catalytic converters henceforth further comprise a particulatefilter that retains the carbon particles, constituting the unburntparticles emitted by the engine. However, a further difficulty hasarisen in the use of these filters, which consists in finding ways toensure that these carbon particles trapped on the filter can be burnt oroxidized as they are deposited, in order to prevent clogging of thefilter.

All particulate filter techniques for diesel engines, used today orunder development, are all faced with the major problem of theincomplete combustion of the particles retained on the filter medium. Infact, under urban use conditions, the temperature reached by the exhaustgases is insufficient to cause this combustion and to limit the ensuingclogging of the filter.

Without chemical assistance, the carbon particles produced by dieselcombustion only begin to oxidize significantly above 500° C., and sincethese temperatures are never reached under urban driving conditions, onbus engines, reliance on a chemical method to lower this oxidationtemperature has become unavoidable.

The absence of chemical assistance gives rise to clogging of the filterwhich, besides the fact that it causes a pressure drop in the engine andhence deteriorated operation of same, causes violent reactionsassociated with the instantaneous combustion of these carbon particlesin excessive concentration in the filter, when engine malfunctioningcauses the violent initiation of this combustion. The rapid combustionof a large mass of particles thereby generates a temperature above 1000°C. and generally leads to destruction of the filter by thermal shock,the temperatures obtained being too high locally.

Several systems are already in use to achieve the continuous oxidationof these particles.

Thus, certain systems propose the installation of a catalytic oxidationmeans upstream of the particulate filter, permitting the conversion ofnitric oxide NO, present in the exhaust gas, to nitrogen dioxide NO₂above 250° C. This technique, called “Continuous Regenerating Trap”(CRT), combines the effects of the particulate filter with the NOoxidation catalyst.

This means consists of a catalytic support on which the catalyst isfixed, the catalyst generally being a precious metal such as platinum orrhodium. The NO₂ produced by the action of said catalyst has theproperty of oxidizing the carbon particles above 250° C. However, thesatisfactory operation of the filter depends on the average temperaturereached and the ratio of particles emitted to the NO₂ formed. To ensurethat the filters operate satisfactorily, this CRT system requiresuniform regeneration, which limits the pressure drop across the filterwhile eliminating the risk of uncontrolled and exothermic regeneration.

This condition is only achieved if the exhaust gases or the combustionchamber are at a temperature above 250° C. during at least 30% of thevehicle running time.

If not, violent reactions are developed, associated with the excessiveconcentration of carbonaceous particles clogging the filter. Thesereactions consist of the excessively rapid combustion of a large mass ofparticles, generally leading to the destruction of the filter by thermalshock, because very high temperatures are reached locally.

A similar means is available constituting a variant of the above, inwhich the catalyst is directly deposited on the particulate filter.However, only some materials suitable for the particulate filter areable to fix metal catalysts. This is the case of cordierite inparticular. In fact, materials of this type are known to be particularlysensitive to the increase in temperature and to thermal shocks. Itaccordingly appears that the sudden increases in temperature which canoccur in a clogged particulate filter can cause irreversible damage tothe filter. This makes it necessary to replace the particulate filterand in general the exhaust device, thereby incurring an absolutelyprohibitive cost.

Other regeneration techniques make use of organometallic additives addedto the diesel, such as cerium, iron, strontium, calcium, and others, inorder to coat the carbon particles formed with the metal oxide of thecatalyst and thereby to oxidize the carbon at lower temperature.

These techniques are suitable for obtaining a similar effect to the oneobtained with NO₂, by catalyzing the combustion of carbonaceousmaterials at temperatures close to 300 or 350° C.

A first drawback of these techniques is the prohibitive cost of theadditives used, compounded by the fact that a supplementary additivedevice must be provided.

A further drawback of these techniques is that they display an evengreater tendency to clogging of the filter and hence to the attendantreactions, because if the temperatures reached in operation are notsufficiently high, the additives present in the carbonaceous materialscontribute to an even faster clogging of the filter medium.

Other techniques have involved testing devices based on supplementaryheating means such as burners, electric resistors and others. Thesesupplementary heating means are only employed when the cartridgedisplays incipient clogging, causing an increase in the pressure drop.Such a regeneration device is put into practice with the engine running,that is to say in the presence of a high exhaust gas flow. Hence such adevice requires high heating capacity to heat the exhaust gases and themass of the filter cartridge to the right temperature simultaneously.

In such a technical context, one object of the present invention is toprovide a method for filtration of exhaust gases (in particular ofdiesel engines) that remedies the drawbacks of the various existingtechniques, by optimizing the filtration of the exhaust gases, forexample of diesel engines, particularly in terms of regeneration of thefiltration means, in order to provide a satisfactory solution to theproblem of clogging of the filtration means by carbon particles.

A further object of the invention is to provide a method for filtrationof exhaust gases incorporating uniform, efficient and continuousregeneration, thereby avoiding any risk of particle accumulation in thefiltration means and hence of uncontrolled regeneration.

A further object of the invention is to provide a method for filtrationof exhaust gases in which the incorporated regeneration does not causeany significant overconsumption of fuel and, in general, does not incurany extra financial cost to the user.

A further object of the invention is to provide a method for filtrationof exhaust gases in which the incorporated regeneration does notdeteriorate engine performance, particularly by pressure drops, causedby the backpressure exerted by the exhaust gases on the engine, due tothe clogging of the filtration device.

A final object of the invention is to provide a filtration device forputting into practice the filtration method according to the invention.

These objectives, among others, are achieved by the present invention,which primarily relates to a method for filtration of exhaust gases,e.g. those emitted by a diesel engine. In this method for filtration ofexhaust gases, all or part of the particles present in said exhaustgases are retained on filtration means and are burnt by the action of acombustion catalyst. This method essentially consists in obstructing atleast a portion of the filtration means as soon as the temperature θg ofthe exhaust gases to be filtered becomes equal to or lower than athreshold temperature θs, so as to limit, indeed to avoid, the coolingof the obstructed portion and to maintain same at a temperature θo thatis equal to or greater than θs, up to the time when θg again becomesgreater than θs, and thereby to permit accelerated regeneration of thisobstructed portion of the filtration means, because the temperatureconditions are better than those that would obtain if this portion ofthe filtration means had not been obstructed.

Hence according to the invention, the exhaust gases are filtered on afiltration means consisting for example of at least two cartridgesdisposed in one casing, one of the two cartridges being short-circuitedwhen the engine is running without load or is idling, in order tomaintain, in the isolated cartridge without flow, a sufficienttemperature to cause a significant continuous regeneration rate, whenthe engine is again running with hot exhaust gases. Each cartridge ispreferably short-circuited in turn so that it is continuouslyregenerated.

In the filtration units thus isolated and maintained at high temperaturein the absence of cold gas, a regeneration process will continue to takeplace slowly thanks to the very slight flow rate that is maintained, butabove all these filtration units will be maintained at an optimaltemperature until the engine is again running and the hot exhaust gasesare again admitted. The regeneration process in these isolatedcartridges can then take place continuously, eliminating any risk ofclogging.

Preferably, the various portions of the filtration means aresuccessively each subjected to the obstruction/regeneration sequence foreach variation of θg between a value v1 that is equal to or greater thanθs, a value v2 equal to or lower than θs, and again a value v3 equal toor greater than θs, v1=or≠v3, so as to permit a uniform and continuousregeneration of the filtration means.

According to a noteworthy feature of the invention, the obstruction of aportion of the filtration means consists in preventing the flow of theexhaust gases in at least 30%, preferably in at least 50%, and even morepreferably in 50 to 75% of the filtration means, this percentage beingexpressed as a percentage by volume.

Preferably, θs=250° C. or 300° C.

Advantageously, the exhaust gases are produced by a supercharged dieselengine and the datum parameters, that is the temperature θg of theexhaust gases and the threshold temperature θs, are given indirectly bythe boost pressure and/or the engine speed and/or the backpressureupstream of the filtration means, the threshold boost pressure beingpreferably equal to 2.5% of the maximum boost pressure of the engine.

According to a preferred embodiment, the filtration means consist of atleast two—preferably at least three—filter cartridges, each equippedwith an obstructor, two of the three cartridges that the filtrationmeans preferably comprise constituting the obstructed portion of thefiltration means when θg≦θs.

According to another of these aspects, an object of the invention is adevice for filtration of exhaust gases comprising at least one catalysismeans, means for filtration of said exhaust gases, disposed in areaction chamber in the path of the exhaust gas stream produced by anengine, said device being characterized in that the filtration meansconsists of at least two assemblies each comprising a catalyst supportadjacent a filter cartridge equipped with a flow obstruction means.

Advantageously, the device comprises a means for recirculating theexhaust gases at the engine intake the operation of which is associatedwith the cutoff of the flow in one or a plurality of the cartridges whenthe engine is not accelerated, so that the increase in backpressuregenerated automatically opens a valve that permits this recirculation ofthe exhaust gases.

According to a preferred feature of the device according to theinvention, each of the filter cartridges has a flow obstruction means,disposed upstream or downstream, controlled by an electronic computerwhich takes account of all the engine operating conditions, in order toisolate at least one cartridge each time the accelerator position is atzero (not accelerated).

In an advantageous embodiment of the device according to the invention,the filtration means consists of at least three cartridges with a flowobstruction means on each of them, controlled by an electronic computerwhich takes account of all the engine operating conditions, in order toisolate, in turn, at least two cartridges when the engine is notaccelerated, and to isolate the cartridge that filtered the gases in thenon-accelerated position, each time the engine is accelerated.

Advantageously, the flow obstruction means disposed on each filtercartridge comprise a small calibrated orifice to maintain a very lowflow rate.

According to an advantageous variant, the device comprises a system forpost-injection of diesel into the exhaust gases, via an atomizer,preferably upstream of the filtration device and the catalysts,controlled by an electronic computer which takes account of all theengine operating conditions, this diesel post-injection system possiblybeing associated with an exhaust gas recirculation system.

In this variant, it may be advisable for the diesel injected to containan organometallic combustion catalyst, supplied or not from a specifictank.

Finally, within the scope of the invention, the device can make use ofknown organometallic additives which are injected by the post-injectionsystem instead of the diesel.

In the preferred embodiment, the filtration means consists of anassembly of at least two filtration units each equipped with anobstruction means controlled by a computer which takes account of theengine operating conditions.

If the device according to the invention comprises more than twofiltration units, each of said filtration units will comprise anobstruction means in order to short-circuit them in turn.

The obstruction means for each of the cartridges used will be placeddownstream of the filtration unit.

According to a variant of the invention, the obstruction means may alsobe incorporated upstream of the filtration unit and the associatedcatalyst.

According to a noteworthy feature of the invention, said filtrationunits will each incorporate a catalyst disk upstream, preferably onmetal support.

The catalyst is a conventional platinum-based oxidation catalyst, inorder to achieve complete oxidation of the hydrocarbons and of the CO.

According to a further variant of the invention, the filtration devicecomprises a system permitting the recycling of the exhaust gases whenthe filtration capacity is reduced, thereby exploiting the increase inbackpressure caused by this restriction to send a portion of theunfiltered exhaust gases into the intake line, through a nonreturnvalve.

According to a variant of the invention, the filtration device comprisesmore than three cartridges and a sufficient number so that, forfull-load running conditions, one of the cartridges is isolated, thiscartridge being reserved for the filtration of the gases at partial loador at idling. The aim is to maintain the filter medium and the catalystof each of the cartridges used at full load, at high temperature. Thecartridges used at idling will be switched with one of the others whenincipient clogging is detected.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from a reading of thedescription that follows, with reference to the drawings which, in anon-limiting manner, show an embodiment of the filtration deviceaccording to the invention and in which:

FIG. 1, according to a preferred embodiment of the invention, shows ageneral view of the system comprising the filtration device with twocartridges, each having an oxidation catalyst on metal support upstreamand, downstream, a valve actuated to completely obstruct, whennecessary, the flow of the gases to be filtered. The filtration deviceis associated with an exhaust gas recycle system with its nonreturnvalve.

FIG. 2 shows a general view of the filtration device comprising acatalyst independent of the filtration units associated with anobstruction system placed upstream.

FIG. 3 shows a general view of the filtration device which incorporatesa diesel injection system.

FIG. 4 shows a variant of the filtration device incorporating threefiltration cartridges.

FIG. 5 shows a general view of the filtration device with all thevariants incorporated in the engine environment.

The system suitable for putting into practice the filtration deviceaccording to the invention is shown in detail in FIG. 1, according to apreferred embodiment. In this system, various mechanical components ofthe vehicle, which do or do not form part of the filtration device, andwhich contribute to the regeneration, collaborate.

Thus the exhaust gases leaving the engine are introduced into the devicevia the nozzle 1, and are then sent to each catalyst disk on metalsupport 2, for filtration on the two filtration cartridges 3, thesecartridges preferably being made of silicon carbide, but possibly alsoconsisting of a filter medium of cordierite or other ceramics. They aredisposed inside a chamber 4 and isolated from it via an insulation 6, toavoid being cooled by the ambient air.

Valves 5 are disposed at the outlet of these filter cartridges in orderto completely isolate each cartridge and totally obstruct the exitchannel. These valves are actuated by air cylinders 7, and the exhaustgases are then sent to the outlet 8.

The device will operate as follows: when the position of the acceleratorreturns to the zero (non-accelerated) position, a position detector notdescribed sends the data to a computer, which alternately actuates eachcylinder to obstruct one of the two cartridges completely and to useonly a single one for these particular operating conditions. The filtermedium of the cartridge obstructed by the valve will thereby remain atthe high temperature that it had reached during the last acceleration,the valve only being disabled to restore it into operation when theengine is again accelerated, hence again at high exhaust gastemperatures. To enable each one of them to be regeneratedsatisfactorily, the same cartridge will be obstructed during an intervalof five to ten minutes, and another alternative will be to measure thebackpressure at idling on the cartridge used and to actuate the switchto the other cartridge when the preset level is reached.

This FIG. 1 also shows the possibility of associating an exhaust gasrecycle system which is automatically actuated, via the line 16, in thedirection of the intake manifold 20, when one of the valves 5 isobstructed, and when the resulting increase in backpressure generates aflow through the valve 17, these conditions corresponding to a zeroaccelerator position hence a low exhaust gas temperature.

The recycling serves to reduce the flow of filtered exhaust gas throughthe cartridge that remains active in these conditions, and hence toreduce its cooling. Similarly, the introduction of hot exhaust gasesmixed with the air entering the manifold at 20, after theturbocompressor 18, through the engine 21, will cause a substantialincrease in the temperature of the exhaust gases discharged through thenozzle 22, possibly raising it during idling from the usual 90 to 100°C. to more than 160° C. at the inlet of the device 23.

The valve 17 is a nonreturn valve with a large cross section or, evenbetter, of the leaf valve type permitting a flow with a few millibar ofoverpressure. The dimensioning of the valve 17 and of the line leadingto the intake manifold 20 is such that this assembly permits recyclingof the exhaust gases amounting to 30 to 60% of the flow under idlingconditions.

As soon as the engine is accelerated, the pressure in the intakemanifold exceeds the backpressure at the filter inlet, obstructing thevalve 17 and automatically interrupting the exhaust gas recycle stream.

In FIG. 2, the variant of the device shown is different in the use of acommon catalyst 14 for the two cartridges 3 and the use of butterflies15, disposed upstream of the cartridges to obstruct the filters insteadof the valves 5, used in the preferred embodiment of FIG. 1.

FIGS. 3 and 5 show the possibility of further having a diesel injectionsystem upstream of the filtration device, actuated from the dataacquired at the pressure sensors 9 and temperature sensors 10, disposedupstream of the filters, the computer adjusting the best strategy tokeep each of the filters perfectly clean, even going as far as to causesupplementary diesel injection through the atomizer 11, supplied withair 12 and diesel 13. These injections are aimed at increasing theexhaust gas temperature when the engine is running at full load, inorder to heat the filter medium to a higher temperature to acceleratethe regeneration rate. These injections are only employed if incipientclogging of the filter is detected.

FIG. 4 is a variant that uses three filter cartridges instead of two,and which permits better maintenance of the temperature in the filtermedium.

In fact, with a device comprising three cartridges, it is possible tointroduce a further variant in the isolation control of each of thecartridges, by using the backpressure and temperature data sent to thecomputer by the sensors according to a strategy described below.

For example, when the accelerator position returns to zero, two of thethree cartridges are obstructed and the gas stream only passes throughone cartridge in this position. The flow is only restored to these twocartridges progressively upon the next acceleration, and thebackpressure data is taken into account to determine the ideal momentfor restoring these cartridges to the circuit. For example, the computerwill trigger the opening of the valve of a first cartridge as soon as abackpressure level of 100 mbar is detected, and the valve of the secondcartridge is only opened if this backpressure level of 100 mbarpersists.

Depending on the type of engine used, this backpressure level may bedifferent from the value of 100 mbar that we have used as an example.

A variant of the strategy described above may be to use the temperaturedata in addition, for example when the temperature θs falls below 250°C. or 300° C., to decide to close one or a plurality of cartridges at agiven backpressure level and independently of the other serviceconditions.

As we have seen, the objective of each of the valves associated witheach cartridge is to be able to isolate them to preserve the hightemperature level obtained during the previous full engine load and toprevent them from cooling upon the following partial or idling load,since this high temperature favors the combustion reactions and there isa major advantage in keeping the valve closed with a low exhaust gasflow to maintain these combustion reactions, which are highly exothermicand which even tend to raise these temperatures. This operation will befeasible by using valves with a small calibrated orifice 24, of lessthan one to a few millimeters, the orifice diameter depending on theengine displacement to allow the necessary flow to pass through.

It should be noted that the possibility of maintaining the filter mediumat higher temperature on at least one of the two cartridges will serveto obtain a far higher nitrogen oxide reduction efficiency on saidcartridge thanks to the reaction of the nitrogen oxides with the carbonthat takes place at higher temperature. With this device, nitrogen oxidereductions of over 30% have been recorded in accordance with theofficial measurement procedures.

Similarly, a higher hydrocarbon reduction efficiency on the officialpollution cycles has been observed thanks to the maintenance of at leastone of the catalyst disks at high temperature for the devices in whicheach cartridge is equipped with its catalyst.

A variant of this control for a system comprising at least threecartridges, and where the dimensioning of each is determined so that thefiltration of the exhaust gases under full load conditions can takeplace on only two of them, will consist in specializing two of thecartridges for full load operation with one reserved for idlingoperation and partial loads, in order to maintain the filter medium andthe catalyst of the cartridges under full load conditions at hightemperature, and to obtain a maximum reduction of all the pollutants. Toenable each of them to regenerate under satisfactory conditions, thecomputer will replace the cartridge used exclusively for idling with oneused at full load, as soon as a backpressure level is detected.

The satisfactory operation of the device associated with the catalyticmeans and of assistances previously described necessarily requires theuse of a diesel with a sulfur content limited to 50 ppm, a level thatwill be generalized from 2005 on.

However, for diesels with sulfur contents higher than 50 ppm, it may beadvantageous to use a device like the one shown in FIG. 3, in which thediesel atomizer 11 is used to inject a solution of organometallicadditive into the diesel from an additional specific tank containingthis mixture.

The temperature gain obtained thanks to the isolation of certaincartridges when the exhaust gas temperatures are too low will serve toobtain satisfactory operation even with the use of such an additive inall situations.

Similarly, this device will be applied to diesel engines of passengervehicles, with the opening and closing of the valves on each cartridgebeing controlled directly from the computer controlling thesecommon-rail direct injection engines. This closure can, similarly to theone described previously, be programmed for idling and for low loads.The temperatures reached on this type of engine will make it possible tomaintain a sufficiently rapid regeneration reaction virtuallypermanently on one of the two cartridges, in order to maintain it at aninsignificant level of clogging.

1. A method for filtration of exhaust gases, whereby all or part of theparticles present in said exhaust gases are retained on filtration meansand are burnt by the action of a combustion catalyst, said methodcomprising obstructing at least a portion of the filtration means assoon as the temperature θg of the exhaust gases to be filtered becomesequal to or lower than a threshold temperature θs, so as to limitcooling of the obstructed portion and to maintain same at a temperatureθo that is equal to or greater than θs, up to the time when θg againbecomes greater than θs, and thereby to permit accelerated regenerationof this obstructed portion of the filtration means.
 2. The method ofclaim 1, wherein various portions of the filtration means aresuccessively each subjected to the obstruction/regeneration sequence foreach variation of θg between a value v1 that is equal to or greater thanθs, a value v2 equal to or lower than θs, and again a value v3 equal toor greater than θs, v1=or ≠v3, so as to permit a uniform and continuousregeneration of the filtration means.
 3. The method of claim 1, whereinthe obstruction of a portion of the filtration means comprisespreventing the flow of the exhaust gases by at least 30% of thefiltration means, this percentage being expressed as a percentage byvolume.
 4. The method of claim 3, wherein the flow of exhaust gases isprevented by at least 50% of the filtration means.
 5. The method ofclaim 4, wherein the flow of exhaust gases is prevented by 50 to 75% ofthe filtration means.
 6. The method of claim 1, wherein θs=250° C. or300° C.
 7. The method of claim 1, wherein the exhaust gases are producedby a supercharged diesel engine and in that the datum parameters, thatis the temperature θg of the exhaust gases and the threshold temperatureθs, are given indirectly by the boost pressure and/or the engine speedand/or the backpressure upstream of the filtration means.
 8. The methodof claim 7, wherein a threshold boost pressure is equal to 2.5% of amaximum boost pressure of the engine.
 9. The method of claim 1, whereinthe filtration means comprises at least two filter cartridges, eachequipped with an obstructor.
 10. The method of claim 9, wherein thefiltration means comprises at least three filter cartridges, and whereintwo of the three cartridges constitute the obstructed portion of thefiltration means when θg≦θs.
 11. A device for filtration of exhaustgases comprising at least one catalysis means, means for filtration ofsaid exhaust gases, disposed in a reaction chamber in the path of theexhaust gas stream produced by an engine, wherein the filtration meanscomprises at least two assemblies each comprising a catalyst supportadjacent a filter cartridge equipped with a flow obstruction means,further comprising a means for recirculating the exhaust gases at theengine intake, the operation of which is associated with the cutoff ofthe flow in one or a plurality of the cartridges when the engine is notaccelerated, so that the increase in backpressure generatedautomatically opens a valve that permits recirculation of the exhaustgases.
 12. The device of claim 11, wherein each of the filter cartridgeshas a flow obstruction means, disposed upstream or downstream,controlled by an electronic computer which takes account of all theengine operating conditions, in order to isolate at least one cartridgeeach time the accelerator position is at zero (not accelerated).
 13. Thedevice of claim 11, wherein the filtration means comprises at leastthree cartridges with a flow obstruction means on each of them,controlled by an electronic computer which takes account of all theengine operating conditions, in order to isolate, in turn, at least twocartridges when the engine is not accelerated, and to isolate thecartridge that filtered the gases in the non-accelerated position, eachtime the engine is accelerated.
 14. The device of claim 11, wherein theflow obstruction means disposed on each filter cartridge comprise asmall calibrated orifice to maintain a very low flow rate.
 15. Thedevice of claim 11, further comprising a system for post-injection ofdiesel into the exhaust gases, via an atomizer, upstream of thefiltration device and the catalysts, controlled by an electroniccomputer which takes account of all the engine operating conditions,this diesel post-injection system optionally being associated with anexhaust gas recirculation system.
 16. The device of claim 15, whereinthe diesel injected contains an organometallic combustion catalyst,supplied or not from a specific tank.